Forty-eight children with childhood Attention Deficit Disorder (ADD) have been treated with a selective or nonselective MAO inhibitor. The major findings to date are that both clorgyline, a selective MAO-A inhibitor, and tranylcyrpomine, a nonselective inhibitor, were effective in decreasing hyperactivity and improving attention, but that l-deprenyl, a selective MAO-B inhibitor, was not effective. There was a significant decrease in urinary MHPG excretion which showed some correlation with behavioral improvement both on clorgyline and tranylcypromine, and on d-amphetamine. However, this decrease in MHPG persisted for several weeks after drugs were stopped while there was immediate behavioral rebound off drug for both amphetamine and for the MAOIs. Biochemical analysis of l-deprenyl subjects has not been completed.